As Trump Slumps, Republican Donors Look to Save the Senate

Mr. Bullock is among the wave of Democratic challengers who raised more in the second quarter their Republican rivals, according to campaign filings made last week. In North Carolina, a relatively unknown Democratic challenger, Cal Cunningham, nearly tripled Senator Thom Tillis’s haul, raising $7.4 million to $2.6 million. In Maine, Senator Susan Collins, the Republican, was substantially out-raised by her Democratic opponent, Sarah Gideon, who collected more than $9 million, compared to only $3.6 million for Ms. Collins.

And in Arizona, Mark Kelly, a Democrat who is a former astronaut and the husband of former Representative Gabrielle Giffords, raised almost $12.8 million. His nearly $24 million in the bank is more than twice as much money as Senator Martha McSally, the Republican incumbent, reported in a race a number of Republican officials fear is slipping away.

Democrats must win a net of three new seats and the White House to take control of the Senate, or four seats if Mr. Trump is re-elected.

“We could be in big trouble in the Senate,” said Fred Zeidman, a Republican fund-raiser in Texas. “If we lose the White House, then we’ve lost everything. The Senate has to be the firewall.”

Senator Mitch McConnell of Kentucky, the Republican majority leader, has been warning donors in dire terms about permanent and systemic shifts that could come about in a fully Democratic-controlled Washington next year: adding Puerto Rico and Washington, D.C., as states, expanding the Supreme Court, and the end of the legislative filibuster, which has previously served as an institutional brake on congressional majorities, according to people who have heard his pitch.